Atmospheric pressure ionization mass spectrometry and ion mobility spectrometry are techniques that detect the presence of, and identify the composition of, ionizable chemical species in a flowing gas stream. This is accomplished by submitting the ions to an ion analyzer where the ions are separated according to characteristic properties of the ions. For atmospheric pressure ionization mass spectrometry, the ion analyzer is a mass filter that uses a combination of electromagnetic fields to determine the charge-to-mass ratios of the ions. For ion mobility spectrometry, the ion analyzer is a drift tube that uses a constant or oscillatory electric field to determine the mobility of the ions. Although the ion analyzer for atmospheric pressure ionization mass spectrometry works under vacuum conditions ( less than 10xe2x88x923 Torr) and the ion analyzer for ion mobility spectrometry works under atmospheric pressure conditions (the definition of mobility only requires that the pressure be greater than approximately 10xe2x88x923 Torr), they share the common feature that the ions are generated under atmospheric pressure conditions. The ions created in an atmospheric pressure ionization source of a mass spectrometer are interfaced to the vacuum of the mass spectrometer through an ion sampling pinhole or orifice.
Morning, et al. in a paper entitled xe2x80x9cNew Picogram Detection System Based on Mass Spectrometer with an External Ionization Source at Atmospheric Pressurexe2x80x9d published in Analytical Chemistry, Vol. 45, No. 6, 1973, pp. 936-943, demonstrated that chemical species can be ionized in air or nitrogen using a radioactive source. Beta particles released by the 63Ni radioactive source create reactant ions that subsequently attach to the chemical species of interest to create product ions. In a paper entitled xe2x80x9cSubpicogram Detection System for Gas Phase Analysis Based upon Atmospheric Pressure Ionization API) Mass Spectrometryxe2x80x9d published in Analytical Chemistry, Vol. 46, No. 6, 1974, pp. 706-710, Homing, et al. further demonstrated that the radioactive source can be replaced with a discharge ionization source. Spangler, et al. in a final technical report entitled xe2x80x9cNonradioactive Source Development for the XM22 Automatic Chemical Agent Alarm and Auxiliary Equipmentxe2x80x9d prepared for the U.S. Army in 1992, demonstrated that the negative ions generated by such a discharge ionization source can differ from those generated by a radioactive source. This difference leads to differences in ionization capabilities for selected groups of ionizable compounds.
From the point of view of building commercial hardware, a discharge ionization source is preferable to a radioactive source because of the liabilities associated with broadly distributing radioactive materials. This preference is causing various manufacturers to replace the radioactive source with other sources such as a corona discharge source. For example, U.S. Pat. No. 4,023,398 describes an atmospheric pressure ionization mass spectrometer that uses a radioactive tritium foil for ionization. The foil was later replaced by a point-to-plane discharge in U.S. Pat. No. 4,121,099. Electronics were provided to apply a high potential between the discharge needle and the pinhole.
U.S. Pat. Nos. 3,626,182 and 4,712,008 further disclose the use of a radioactive source for ionization of sample in a linear ion mobility spectrometer. The radioactive 63Ni foil occupies the inner diameter of a guard ring that is otherwise used to electrically bias the cell. A similar source is disclosed in U.S. Pat. No. 5,420,424 to ionize sample in a transverse-field ion mobility spectrometer. This source was replaced by a discharge ionization source at the University of Toronto and evaluated by Karasek and Kim of the University of Waterloo. In their report entitled xe2x80x9cStudy of Technology Relating to Plasma Chromatography Sensing Tubesxe2x80x9d that was submitted to the Canadian Government (DREV) in 1980, Karasek and Kim noted that an ionization source of the type described in U.S. Pat. Nos. 4,023,398 and 4,121,099 does not work in an ion mobility spectrometer. Insufficient ions passed through the pinhole to produce a measurable signal in the ion mobility spectrometer. Later work by Spangler, et al., as described in the final technical report entitled xe2x80x9cNonradioactive Source Development for the XM22 Automatic Chemical Agent Alarm and Auxiliary Equipmentxe2x80x9d submitted to U.S. Army in 1992, showed that the ion current could be increased if the pinhole of U.S. Pat. Nos. 4,023,398 and 4,121,099 was eliminated and replaced with a grid. Operation of the resulting point-to-grid discharge, however, was hampered by the need to use excessively high potentials to create positive ions (leading to burnt electrodes), and incorrect ionization chemistry for the negative ion mode of operation. The incorrect negative ion chemistry was attributed to secondary reactions that occurred in the hot plasma.
Taylor, et al. in U.S. Pat. No. 5,684,300 replaced the point-to-grid discharge with a point-to-target discharge. Their target electrode was the internal surface of a bias ring that was otherwise used to bias the IMS cell. Consistent with the observation of Spangler, et al., they found that considerably higher potentials were needed to establish and maintain the discharge. Unlike Spangler, et al., Taylor, et al. had the ability to generate higher potentials (up to 10 kilovolts), and used electrodes more tolerant towards the ion energies produced by these potentials. Their source was more reliable, but pumped an excessive amount of energy (albeit for a short period of time) into the discharge gap. The excessive amount of energy lead to a requirement for delayed sampling of ions to preserve ion chemistry.
Finally, Spangler et al. in a presentation entitled xe2x80x9cLow Energy Glow/Corona Discharge Ionization Source for Ion Mobility Spectrometry,xe2x80x9d delivered to the 7th International Conference on Ion Mobility Spectrometry, Hilton Head, SC in 1998, disclosed a point-to-point discharge ionization source that removed the previous limitations associated with negative ionization. The discharge was generated between two electrodes positioned across the diameter of the IMS cell. Because the discharge was a dc discharge produced by a power supply with limited current producing capabilities, the discharge was unstable and generated severe noise in the ion mobility spectrum.
Hyne in U.S. Pat. No. 3,848,202; LeMay in U.S. Pat. No. 3,940,710; and McLellan in U.S. Pat. Nos. 4,412,333, 4,748,635 and 4,556,981 disclose a three-electrode discharge ionizer that is more stable than a two-electrode discharge ionization source. The third electrode pre-ionizes the gas between the anode and cathode. The configuration is similar to that disclosed by LaFlamme in an article entitled xe2x80x9cDouble Discharge Excitation for Atmospheric Pressure CO2 Lasers,xe2x80x9d published in The Review of Scientific Instruments, Vol. 41, No. 11, 1970, pp. 1578-1581, and is a variation of the spark gap commonly used to control the operation of lasers (see U.S. Pat. No. 4,481,630). The idea behind the arrangement is that it is easier to break down a narrower gap than a wider gap (a consequence of Paschen""s curve); and that once charge is created in a gap, a lower electrical potential is needed to break down the gap. Thus if a third electrode is placed in close proximity to the cathode and biased with the same potential as the anode, the gap between the third electrode and cathode breaks down first, followed by a discharge across the main discharge gap between the anode and cathode. This concept is utilized in U.S. Pat. No. 5,684,300 where the third electrode is biased with a potential opposite the corona discharge electrode to control the duration of the discharge, as well as the quantity of the ions generated. Other possible functions for the third electrode are to serve as a second anode or cathode (depending on polarity) as described in U.S. Pat. No. 5,684,300, or to act as a control electrode similar to that described by Kaibyshev, et al. in a paper entitled xe2x80x9cEffect of a Third Electrode on a Low-Voltage Arc,xe2x80x9d published in Soviet Physics Technical Physics, Vol. 20, No. 2, 1975, pp. 203-207.
The present invention discloses a simple low energy discharge ionization source for atmospheric pressure ionization mass spectrometry and ion mobility spectrometry that is well suited for operation from a constant source of high potential, wherein the ion current conducted through the discharge gap is controlled by ballast resistors, and the operation is stabilized against random fluctuations using a third pre-ionization/control electrode. In addition to being a gas phase ionizer, the source can also be configured as an electrospray ionizer. The simplicity of the source permits construction of a rugged, reliable and inexpensive ionizer.
The present invention provides an atmospheric pressure ionization mass spectrometer or ion mobility spectrometer that does not require the use of a radioactive source for ionization, yet preserves and/or enhances the sensitivity and specificity of ionization compared to a radioactive source.
The present invention also provides an ionization source that functions similarly when used in combination with atmospheric pressure ionization mass spectrometry and ion mobility spectrometry to provide similar ionization capabilities.
The present invention also provides a dc discharge ionization source for atmospheric pressure ionization mass spectrometry and ion mobility spectrometry by applying a substantially constant potential across an anode and cathode, such discharge being automatically initiated when the proper potentials are applied to the electrodes.
The present invention also provides a discharge ionization source that uses simple electronic circuitry for its operation and control; the main discharge and/or pre-ionization currents being controlled by ballast resistors.
The present invention also provides a pulsed discharge ionization that produces sufficient ionization current for atmospheric pressure ionization mass spectrometry and ion mobility spectrometry, while still enabling control of the electrical energy dissipated by the discharge.
The present invention also provides an ionization source that consumes low power and is convenient and safe to use in detectors and instruments using such a non-radioactive source.
The present invention also provides an atmospheric pressure ionization mass spectrometer or ion mobility spectrometer that utilizes a discharge ionization source to ionize chemical species in a sample matrix presented for analysis.
The present invention also provides an atmospheric pressure ionization mass spectrometer and ion mobility spectrometer that can be used in combination with gas chromatography, liquid chromatography, or electrophoresis.
The present invention relates to an improved discharge ionization source for atmospheric pressure ionization mass spectrometry and ion mobility spectrometry that utilizes two or more electrodes. Two of the electrodes serve as the anode and cathode for the main discharge, while the other electrode(s) may serve as control electrode(s). One of the secondary electrode(s) may be located near the cathode to serve as a pre-ionizer of gas in the discharge gap. The discharge gap is biased by connecting the anode and secondary electrodes to a common source of high potential through at least one ballast resistor, while connecting the cathode to the low side of that potential (that may also serve as a floating ground). The value of the ballast resistor(s) are adjusted to provide more or less current flow through each electrode; but in the presence of an active discharge, the main current flow is preferably through the anode and cathode. When using two electrodes, the dc potential is initially increased to exceed the breakdown potential of the discharge gap, and then reduced to support continuous operation of the discharge. When using three or more electrodes, the dc potential is adjusted to exceed the breakdown potential of a short discharge gap, while at the same time supporting continuous operation of a discharge across a wider discharge gap. The discharge in a wider discharge gap can thus be initiated by pre-ionization in the shorter discharge gap. The discharge may be pulsed by raising and lowering the potential applied to the cathode relative to the anode and secondary electrodes. The pulse electronics are simple, including a transistor switch capacitively coupled to a negative diode clipping circuit and the cathode. The ions generated by the discharge are introduced into the mass or ion mobility spectrometer for analysis by providing a secondary extraction potential between the ionization source and the ion entrance aperture (i.e., sampling pinhole for an atmospheric pressure mass spectrometer, or shutter grid or ion entrance slit for an ion mobility spectrometer) for the spectrometer.
In another aspect the present invention relates to a method of ionizing chemical species using a discharge ionization source including introducing the chemical species into a volume of gas or liquid, flowing the volume of gas or liquid through a reactor volume housing the discharge ionization source, providing the necessary potentials to the ionization source to assure ionization of the gas or liquid plus the chemical species, extracting the ions from the discharge ionization source by applying an accelerating potential between the ionization source and the entrance aperture of the ion analyzer, delivering the ions to the ion analyzer for mass or mobility analysis, exhausting the unused sample from the reactor volume, and analyzing the spectrum or spectra produced by the ion analyzer to identify and quantitate the amount of sample species present in the ionizer.
Some of the benefits and advantages of the present invention include a reduction in the high potential needed to sustain the operation of the discharge ionization source, simplification of the electrical circuitry required to power the discharge ionization source, improved control of the electric current flowing through the discharge gap, arrested operation to prevent formation of an arc discharge, a longer discharge gap so that more ions are available for analysis, reduced formation of reactive neutrals in the discharge gap, and the possible elimination of the need for a shutter grid in ion mobility spectrometry.
Additional features of the invention will be set forth in the detailed description that follows. These will become apparent to those skilled in the art of ion mobility spectrometry, mass spectrometry and/or discharge ionization sources, and may be learned by those practicing the invention.
The present invention also relates to a discharge ionization source that can be used in combination with atmospheric pressure ionization mass spectrometry or ion mobility spectrometry for the detection of ionizable chemical species in air (or other carrier gas). More particularly, the present invention relates to an improved discharge ionization source that uses solid state circuitry for its operation and control. Inductive coupling of the electronics to the discharge gap is not required. Two or more electrodes are used to sustain the discharge. Two of the electrodes are the main electrodes serving as the anode and cathode. A third electrode may be added to pre-ionize the gas, and thus automatically and more reliably initiate and start the discharge. Other functions for the third electrode include stabilizing the discharge and switching between gas and electrospray ionization. The discharge may be operated as a continuous discharge or as a pulsed discharge. These modes of operation are selected by providing the appropriate control potentials (e.g., pulsed or dc) to the cathode. The cathode and/or the anode may also be used to control potential. The cathode is particularly advantageous because electron emission (a necessary requirement to sustain a discharge) occurs from this electrode.